1. Field of the Invention
The present invention relates to a switching power supply, and a drive system for a DC/DC Converter.
2. Description of the Related Art
A step-up type of switching power supply for generating voltage higher than an input voltage is widely used in various electronic devices. This step-up type of switching power supply is provided with a switching element, and an inductor or a transformer, and by putting the switching element ON or OFF in a time-division way, a back electromotive force is generated in the inductor or the transformer, and input voltage is stepped up and outputted.
Among such switching power supplies, two types of insulating-type DC/DC converters that use the transformer are known: a self-excited type and a separately excited type. These types are selected in accordance with a characteristic required of the switching power supply, such as range of output voltage, or the like. In the insulating-type of DC/DC converter, when a switching transistor is OFF, a current flows on a primary side of the transformer, and energy is stored in the transformer. When the switching transistor is OFF, energy stored in the transformer on a secondary side of the transformer is transferred to an output capacitor, via a rectifier diode, as a charging current, and the output voltage rises.
In this type of separately excited DC/DC converter, when an overcurrent flows due to a short-circuit of a load, or the like, since the transformer is saturated, or reliability of the switching transistor is effected, there are cases in which an overcurrent protection circuit is provided. For example, Patent Document 1 discloses technology in which the current on the primary side of the transformer is monitored to detect an overcurrent state.
Patent Document 1: Japanese Patent Application, Laid Open No. 2002-374671
However, in the technology described in the abovementioned document, in which the current on the primary side of the transformer is monitored, when a load suddenly short-circuits, there have been cases in which current flowing on the primary side of the transformer suddenly rises at a speed exceeding response speed of a voltage comparator, so that overcurrent protection response is delayed, and the overcurrent flows in the switching transistor.